1. Field of the Invention
The present invention relates to a liquid crystal driver used for correcting tilt of an optical head used in an optical disk drive.
2. Description of Prior Art
Recently, it is proposed to correct tilt in an optical head in an optical disk drive with a liquid crystal element in order to correct aberration (Japanese Patent laid open Publications 10-79135/1998 and 11-3531/1999). The tilt is an inclination of the optical axis of an optical head relative to an optical disk. Electrodes in the liquid crystal element are provided as a plurality of divided areas of predetermined shapes, and voltages applied to the areas are controlled for tilt correction by changing the phase difference of a transmitting laser beam. Spherical aberration of object lens is a problem for higher density recording of optical disk when a short wavelength laser is used for a lens of high numerical aperture (NA). Spherical aberration is also corrected with the liquid crystal element.
When tilt correction is performed only in one direction, the liquid crystal element has divided areas of generally long areas and the voltages applied to the areas are controlled. When tilt correction is performed in two directions, the liquid crystal element has divided areas of generally fan-like shapes extending in two directions, and a central area around the optical axis is provided as a common area. In this case, the voltages applied to the areas are controlled similarly to the correction in one direction.
In order to apply voltages, an operational amplifier or a pulse width modulation circuit is used generally. When tilt correction is performed in one direction and in two directions, as mentioned above, a driver circuit is provided for each of the areas in the liquid crystal element.
As to the driver circuit, it is necessary to provide a driver circuit for each of the divided areas in the liquid crystal element. Then, the circuit scale of the driver circuit becomes large. Further, a large number of connections for driving the liquid crystal element are needed in an interface between the optical head with the liquid crystal element and the circuit board.
Further, when tilt correction is performed in two or more directions, it is desirable to correct the tilt for each direction independently. However, it is troublesome to determine voltages applied to the areas except the common area for each direction by taking into account the voltage applied to the common area. Recently a digital signal processor is used to determine the applied voltages, and the number of the steps becomes large in the software processing in the digital signal processor.
In a prior art optical head, a liquid crystal element is mounted not on an actuator which is a moving component, but on a fixed component. This is adopted in order to improve sensitivity on focusing and tracking by making the optical head compact and by using a light-weight actuator. Tilt caused by inclination of the object lens in radial and tangential directions are corrected. However, when the object lens is moved in the radial direction, it is a problem that the optical axis of the object lens is shifted relative to the liquid crystal pattern, so that performance of aberration correction is deteriorated.